Conventionally, a semiconductor device having an IGBT is well known to be utilized in an electronic equipment such as an industrial motor (for example, see Patent Literature No. 1).
Specifically, in the semiconductor device, a base layer is formed in a surface portion of a semiconductor substrate having a drift layer with a N-conductive type, and a carrier storage layer (i.e., CS layer) is formed between the base layer and the drift layer. Multiple trenches are formed to penetrate the base layer and the CS layer. Each trench is filled with a gate insulation film on a sidewall of the trench and a gate electrode on the gate insulation film. An emitter region having a N+ conductive type is formed in a surface portion of the base layer to contact with the trench.
A collector layer having a P+ conductive type is formed on a back side of the semiconductor substrate. An emitter electrode is formed on a front side of the semiconductor substrate to be electrically connected to the base layer and the emitter region. A collector electrode is formed on the backside of the semiconductor substrate to be electrically connected to the collector layer.
In the semiconductor device, when a voltage lower than the collector electrode is applied to the emitter electrode, and further, a voltage equal to or larger than a threshold voltage Vth of an insulation gate structure is applied to the gate electrode, an inversion layer (i.e., a channel) having the N conductive type is formed at a portion of the base layer contacting with the trench, and an accumulation layer of an electron is formed at a portion of the drift layer and a portion of the CS layer contacting with the trench. Further, the electron is supplied to the drift layer via the accumulation layer and the inversion layer from the emitter region, and the hole is supplied to the drift layer from the collector layer. Thus, the resistance of the drift layer is reduced by conductivity modulation, so that the device becomes an on state. At this moment, since the hole accumulated in the drift layer is restricted by the CS layer from discharging to the emitter electrode via the base layer, the on state voltage is reduced.
However, in the semiconductor device, although the on-state voltage is reduced by the CS layer, a difficulty arises such that a switching controllability is reduced when switching from the off state to the on state since the CS layer is formed. Here, the off state indicates a feature such that current does not flow between the collector electrode and the emitter electrode. The on state indicates a feature such that the current flows between the collector electrode and the emitter electrode.
Thus, when a predetermined voltage is applied to the gate electrode, the gate potential increases gradually. Then, when the gate potential becomes equal to or larger than the threshold voltage Vth, the current starts to flow between the collector electrode and the emitter electrode. In this case, the hole supplied to the drift layer is restricted by the CS layer from discharging from the emitter electrode, and the hole is drawn to the accumulation layer. Here, since the hole is easily accumulated near the CS layer, the hole is easily accumulated at a portion of the accumulation layer disposed near the CS layer. The gate potential is changed by the hole accumulated at the portion, and therefore, the switching controllability is reduced.